To date, there have been few studies involving the efficacy of combinations of antiretrovirals in HIV-infected children who are antiretroviral experienced and stable. And currently, the duration of efficacy of continuing stable therapy in children is undefined. In addition, the indications for timing of changes of antiretroviral therapy in children are not clear. It is therefore important for the durability and efficacy of new therapeutic combinations of antiretrovirals be identified for children. The goal of this trial is to compare the efficacy of 4 different combinations of antiretroviral agents to reduce viral replication by 2.2 logs or more at 12 weeks and the ability of the combination to maintain viral suppression to "undetectable" levels in children who are clinically stable and antiretroviral experienced. The 4 combinations to be tested in this multicenter trial involving 240 clinically stable, HIV-infected children between the ages of 4 months and 17 years include: Arm A: d4T, nevirapine, ritonavir Arm B: d4T, 3TC, nefinavir Arm C: d4T, nevirapine, nelfinavir Arm D: d4T, 3TC, nevirapine, nelfinavir